1. Field of the Invention
The present invention relates to a common mode filter that significantly reduces common mode noise that propagates through a transmission line.
2. Description of the Related Art
In order to significantly reduce common mode noise that is to propagate through a differential transmission line, a common mode choke coil has been conventionally used. The common mode choke coil, as described in Japanese Unexamined Patent Application Publication No. 2003-133135, for example, includes two coils that are configured so that a magnetic field generated in each of the coils is canceled out by a differential mode signal (normal mode signal) and the magnetic field is increased with respect to a common mode signal (common mode noise).
In general, such a common mode choke coil is required that each coil has a large inductance value and that a coupling coefficient between the coils is large.
However, as the inductance value of a coil is increased and a coupling coefficient between coils is increased, a self-resonant frequency is decreased and the transmission loss of a signal is also increased. Therefore, it is difficult to use such a common mode choke coil for a differential transmission line that is used in a high frequency band and required to have a small transmission loss, like a differential transmission line for a high speed interface such as USB or HDMI (registered trademark).